The present invention relates to a plasma torch, used for heating molten steel, capable of suppressing the melting loss of the anode electrode and of extending the life thereof.
A slab has heretofore been conventionally produced by (i) transferring a molten steel from a ladle to a tundish, (ii) pouring the molten steel into a mold through a submerged nozzle provided in the bottom portion of the tundish, (iii) cooling the poured molten steel with the mold and a water spray through coolant water nozzles provided to a holding segment, whereby the molten steel is solidified, and (iv) withdrawing the resultant slab with pinch rolls at a given rate.
However, the molten steel transferred to the tundish most likely loses heat to the atmosphere. As a result, the temperature of the molten steel within the tundish becomes lower than a standard temperature, during casting, when the casting time is prolonged due to a large capacity of the ladle, or when the overheating temperature of the molten steel is restricted due to the steel type.
The submerged nozzle for pouring a molten steel into a mold is skulled, or separation of impurities (e.g., inclusions) is hindered due to the temperature lowering, and the quality of the slab is impaired. When the steel temperature is extremely lowered, the casting operation itself may be interrupted.
As described in Japanese Patent Publication No. 3-42195, the certain countermeasures have been taken. For example, a pair of plasma torches (each having an anode electrode and a cathode electrode) is arranged above the surface of a molten steel within a tundish, and a plasma arc is produced between the plasma torches and the molten steel to heat the molten steel with the heat thereof. Moreover, argon gas and CO gas are used as the gas for the plasma to increase the arc voltage, and the output of the plasma arc is thus increased.
Furthermore, as described in Japanese Patent Publication No. 6-344096, the anode electrode of plasma torches is arranged above the surface of a molten steel within a tundish, and an electrode constituting the cathode is immersed in the molten steel. Thus, a plasma arc is produced on the surface of the molten steel from the anode electrode to heat the molten steel.
However, as described in the methods of heating molten steels described in Japanese Publication Nos. 3-42159 and 6-344096, the tip ends of the plasma torches are worn out due to melting losses or wear, and the lives of the plasma torches are very short.
The surface of the anode electrode of the plasma torches during heating the molten steel is locally melt lost or worn out by the heat of the plasma arc or radiation heat of the molten steel and by the splashes or the like of the molten steel caused by the plasma arc, the argon gas for forming plasma, or the like.
As a result, recesses and protrusions are formed on the surface of the electrode, or the tip end of the anode electrode becomes thin, and the tip end deforms outwardly to form a so-called protruded portion (or protrusion).
When the protruded portion is formed, a plasma arc concentrates thereat to increase a heat load on the protruded portion, and the surface temperature exceeds the melting point of the electrode material.
Furthermore, because the molten steel is heated by applying a current as large as from 1,000 to 5,000 A so that a plasma arc is continuously produced on the molten steel surface, concentration of the plasma arc in the protruded portion and melting loss (e.g., wear) of the protruded portion are repeated. As a result, the melting loss (wear) drastically proceeds. The phenomenon becomes significant when DC twin-type plasma torches are employed.
In addition, when splashes of the molten steel are produced, the base metal sticks to the anode electrode and the outer cylinder. The base metal sticking thereto generates a plasma arc that is a so-called side arc in a space other than the one between the anode electrode and the molten steel surface.
In particular, when materials having melting loss resistance and wear resistance are used for the anode electrode and outer cylinder, a side arc tends to be generated depending on the electric resistance, the electric conductivity, and the like of the materials.
When a side arc is generated, the surface of the anode electrode, or the front end (outer cylinder) or the like is opened, to leak water, and the life of the anode electrode is greatly shortened.
Consequently, the heating treatment cost of the molten steel rises, and problems such as the time required for replacing the plasma torches, the deterioration of the quality of the slab caused when the heating becomes impossible and destabilization of the casting operation caused by skulling of the submerged nozzle, arise.
Exemplary embodiments of the present invention take into consideration the above-described situation. Accordingly, one of the objects of the present invention is to provide a plasma torch for heating a molten steel that prevents the melting loss and wear of an anode electrode caused by heat produced in the anode electrode and splashes, that suppresses generation of a side arc, that has a longer life, and that stabilizes the casting operation and improves the quality of the slab.
A plasma torch according to an exemplary embodiment of the present invention can be used for heating a molten steel, and may achieve the above-described object is provided. In particular, the plasma torch may be used for heating a molten steel and can have an outer cylinder. Such cylinder by be composed of a double tube the bottom of which is clogged annularly, and a bottomed cylindrical anode electrode that is installed within the outer cylinder with a gap existing between the anode electrode and the inside of the double tube. For the plasma torch, pure copper is preferably not used as the electrode material, the material has a softening point exceeding 150xc2x0 C., and the ratio of an electric conductivity D of the anode electrode to an electric conductivity N of the outer cylinder satisfies the following formula:
0.2xe2x89xa6D/N less than 1.0. 
Because a material having a softening point higher than that of pure copper is preferably used for the anode electrode, melting loss or wear of the tip end, and the like, caused by the heat of a plasma arc, the radiation heat and splashes of a molten steel, and the like, can be suppressed. Moreover, at approximately the same time, bulging of the anode electrode caused by cooling water pressure is suppressed so that the surface is kept substantially smooth, and melting loss caused by the concentration of a plasma arc can be prevented.
Furthermore, the softening of the surface of the anode electrode facing a molten steel may be suppressed so that the melting loss and the wear caused by splashes can be prevented and generation of a side arc caused by the electric conductivities of the anode electrode and the outer cylinder can also be prevented.
When the D/N ratio becomes less than 0.2, the electric conductivity of the outer cylinder becomes too high in comparison with that of the anode electrode, and a side arc is generated from the anode electrode to the outer cylinder.
When the D/N ratio becomes 1.0 or more, problems such as deterioration of the melting loss resistance and wear resistance caused by a decrease in the softening point of a material used for the anode electrode, or lowering of the electric conductivity of the outer cylinder arise. As a result, the operation is destabilized due to poor ignition.
In addition, the softening point of a material is a temperature at which the hardness of the material is lowered to 35% of the maximum hardness of the material when the material is heated at the temperature for 2 hours.
In order to extend the life of the anode electrode, the heat conductivity and electric conductivity of the material of the electrode has been taken into consideration, as described in Japanese Patent Application No. 2001-179246. However, a material having a high heat conductivity is preferable to improve the heat resistance in view of a material design of the anode electrode; moreover, a material having a low electric conductivity is preferred to improve the arc resistance. However, selection of a material compatibly showing heat resistance and arc resistance has been difficult in the past.
However, using a material showing low electric conductivity while maintaining heat conductivity, a long life plasma torch has been attained. As a result, the life of a plasma torch can be greatly improved in comparison with a conventional torch by restricting the ratio of an electric conductivity of the anode electrode to an electric conductivity of the outer cylinder to a specific range.
Furthermore, the flow rate of an argon gas for forming plasma supplied to the plasma torch should preferably be from 300 to 1,000 NL/min.
Because an ionized argon gas-containing argon gas flow that encloses the tip end of the electrode and that proceeds from the electrode toward the surface of a molten steel is formed between the electrode and the molten steel surface, turbulence of the plasma arc from the electrode to the molten steel surface can be removed, and generation of a side arc can be prevented.
When the flow rate of the argon gas becomes less than 300 NL/min., an ionized argon gas flow may be weakened, and an argon gas flow covering the periphery of the electrode is not formed, whereby a side arc is likely to be generated.
When the flow rate of the argon gas exceeds 1,000 NL/min., the effect of stabilizing a plasma arc usually cannot be expected, and the argon gas flow forms splashes of a molten steel to shorten the life of the electrode.
All cited references are hereby incorporated herein by reference in their entireties.